Florida Keys or Sanibel?

This is where Sanibel pulls ahead. Its Gulf shore is sand almost from end to end, sloping gently into calm water, and shelling at Bowman's Beach and Lighthouse Beach gives a beach day something to do. The drawbacks are occasional red tide and water that is greener than it is blue.

The Keys have warmer water in winter but far less beach, because the reef stops sand building up on the shore and leaves mangrove, rock and seagrass flats. Bahia Honda and Sombrero Beach are the real exceptions, and most swimming here is done from boats and docks.

Both serve the same Gulf staples, grouper, stone crab and shrimp, but the Keys offer more. Key West adds Cuban bakeries, café con leche and a handful of serious dining rooms in Old Town. Up the chain, marina restaurants will cook the fish you caught, and conch fritters and Key lime pie are the local signatures.

Sanibel's breakfast places and porch restaurants along Periwinkle Way are pleasant but much alike, and anyone wanting variety drives to Fort Myers or Naples.

Key West gives the Keys a real if compact round of sights. There are the conch houses and porched mansions of Old Town, Hemingway's house and the Truman Little White House, and a ferry ride away stands the huge brick Fort Jefferson in the Dry Tortugas. Together they fill two days.

Sanibel's sights are small and stick to the island's theme: the Bailey-Matthews Shell Museum, the 1884 lighthouse and a village of pioneer buildings. They are done in a morning.

There is no contest here. Key West's evening runs from the sunset crowd at Mallory Square into open-sided bars, rooftop decks and live bands along Duval Street until very late, with a long-established gay scene and drag cabaret at its lower end. It is a place for bar-hopping rather than clubbing, and outside Key West the Keys go quiet early.

On Sanibel the evening means dinner after sunset, a few cheerful bars on Captiva, and an island asleep well before midnight. For a later night you drive to Fort Myers Beach.

Both have mangrove tunnels to paddle and tarpon to fish for, but the Keys also have a coral reef running the length of the chain. You can snorkel and dive at John Pennekamp, Looe Key and the Dry Tortugas, explore wrecks off Key Largo, and fish the Islamorada backcountry, one of the best-known flats fishing grounds anywhere. The coral itself is patchy, so the fish are often the bigger draw.

Sanibel's strength is its birds. The Ding Darling refuge brings spoonbills, pelicans and alligators close on a winter morning, and Corkscrew Swamp is an hour inland. Its nature is gentler and mostly on land, with nothing under the water to match the Keys.

Neither is a strong base for outings, but they offer different kinds. From the Keys the choice is Miami, about an hour and a half from Key Largo, the drive along the Overseas Highway, and the boat from Key West to Fort Jefferson.

Sanibel has more variety close by. Captiva is over a bridge, Fort Myers has the Edison and Ford Winter Estates, Naples is an hour south, and boats run through Pine Island Sound to Cabbage Key, Cayo Costa and Boca Grande.

Sanibel is the easier choice for families. Shallow, gentle water, hours of shelling, safe flat bike paths and the touch tanks at the shell museum suit young children well, though teenagers may run out of things to do after a few days.

In the Keys, families do best in Marathon and the Upper Keys, which have the sea turtle hospital, the dolphin centres and the glass-bottom boats at John Pennekamp. But there are fewer beaches, the drives are longer, and Key West's Duval Street is aimed at adults.

You need a car for both, and both clog up with traffic in the winter season, so the real difference is scale. The Keys stretch about 180km along a single road of bridges, where one hold-up stops everything, though Key West's Old Town is easy to walk once you arrive.

Sanibel is one small, flat island where shared paths make the bicycle the natural way between beach, refuge and shops. Its bottlenecks are the causeway and Periwinkle Way.

Neither is somewhere to shop, and both offer much the same resort wear, shells and coastal art. Key West's Duval Street adds cigar rollers, hot sauce and Key lime products, with galleries and sea-glass jewellery on the Old Town side streets. Sanibel keeps to small plazas on Periwinkle Way and in Captiva village, and for anything more you go to the outlet centres on the mainland.

Both are expensive, and neither has a real budget version. Sanibel is a little easier on the wallet. Cottages and condos with kitchens are the usual places to stay, staying over the causeway in Fort Myers is an easy fallback, and the beaches and bike paths cost almost nothing.

The Keys charge some of the highest winter room rates in the country, and charter fishing, dive trips and waterfront cocktails add up quickly. Marathon and the Lower Keys are the cheaper places to stay.

Neither is a place to go for theatre or concerts, but Key West has more on. The Tennessee Williams Theatre and a few small playhouses run winter seasons, and Duval Street has live music from early afternoon until late, mostly acoustic sets and cover bands, plus drag cabaret.

Sanibel offers a modest winter programme of chamber concerts and community theatre. For bigger shows you drive to the performing arts hall in Fort Myers.

Both are relaxing because of their pace rather than their spas, and neither has a bathing tradition. In the Keys the calm comes in pockets: waterfront cottages in the Middle and Lower Keys and resort spas around Islamorada, with livelier Key West at the end of the road. Sanibel is quiet all over, with no high-rises, slow roads and early evenings, and the larger resorts on Sanibel and Captiva have full-service spas.